Titan: swimming in the rain

Signs of lakes, flash floods, storm clouds and drizzle seen on Saturn’s moon.

What's the weather like on Titan, Saturn's largest moon? New research this week suggests it is pretty wet. Papers published in Nature show evidence of a light drizzle, and forecast the potential for occasional flash floods from storm clouds. Meanwhile, recent radar images from the Cassini spacecraft have revealed a land of lakes in Titan's northern hemisphere.

When Cassini reached Saturn's moon Titan in 2004, researchers discovered a number of interesting geological features, including apparent river canyons, and immediately began to debate how they were formed. Clues have so far come in dribbles, with some studies showing "probable" rain, and others seemingly ruling out large bodies of water, at least in the bits of the moon seen thus far (see Titan disappoints ocean hunters).

But Cassini's seventeenth close flyby of the moon, which happened on 22 July and brought the craft into more northern regions, provides the most solid evidence yet that there are big lakes the size of North America's Great Lakes on Titan.

It's raining methane

“Lakes? We won't be 100% sure until we can dip our toes in one.”

Jonathan LenineUniversity of Arizona

Titan is often compared to Earth: larger than the planet Mercury, it has a dense atmosphere, and shows features such as clouds, hills, dunes, river channels and now lakes. But the atmosphere is ten times thicker than Earth's, and much colder: at 179 C on the surface, it is methane, not water, that dominates Titan's weather.

When the Huygens probe dropped through Titan's haze on 14 January 2005, there was probably a light methane drizzle. So say Tetsuya Tokano and colleagues who publish their results in Nature today1. "We analysed data on the temperature, pressure and methane concentration profiles as the probe descended," explains Tokano, from Cologne University's Institute for Geophysics and Meteorology, Germany.

They found two distinct layers of cloud: one holding methane ice, and then underneath a liquid methane/nitrogen mix that drizzled to the surface. "But drizzle cannot produce the geological-like features seen on Titan," says Tokano. "We would need much heavier rainstorms at other times."

Such storms are predicted by Ricardo Hueso and Agustín Sánchez-Lavega from the University of the Basque Country in Bilbao, Spain2. "We simulated methane clouds under a range of conditions to try and reproduce cloud observations at Titan's south pole," says Hueso. Their models predict bouts of heavy rainfall.

"The model is quite complete, though I'm not sure it is optimized for polar conditions," comments Pascal Rannou, of the University of Versailles in St Quentin, France, who has worked on models of methane circulation around Titan.

Land of lakes

These scientists were looking at Titan's south pole, where it is now summer and where most clouds have been observed. In the wintry north pole, where liquid methane would evaporate less quickly from the colder surface, Cassini's newest radar pictures seem to show liquid methane lakes. The images were taken at 950 kilometres from the surface, the closest Cassini can get to Titan's atmosphere.

Images of dark lake-shaped patches and channels are not in themselves conclusive. But a variety of other evidence convinced the scientists. "For two days we asked the team to talk themselves out of seeing these pictures as lakes, but no one was able to," says Steve Wall from NASA's Jet Propulsion Laboratory in Pasadena, California.

The darkness of the radar image indicates that the surface of the patches is very smooth, a good indication of liquid. And whereas the darkness could be explained by chemicals derived from the atmosphere snowing down on the ground, this wouldn't create a patch with very sharp edges; a body of liquid would.

The patches also look to be surrounded by little drainage shelves, notes Jonathan Lunine, a planetary scientist from the University of Arizona, Tuscon. This indicates that a liquid has been eroding the sides as the lake level moved up and down over time. Finally, the microwave background suggests that the lakes are warmer than their surroundings, so are probably liquid methane rather than ice, adds Lunine.

"We won't be 100 sure until we can dip our toes in one, but all of the tests we could perform now come up positive for lakes," Lunine concludes.

What will happen to Titan's weather in the future? Because it takes Titan 29.5 Earth years to orbit the Sun, Cassini will have to hang around to spot any seasonal variations. "We need to 'follow the methane' and find out where it comes from and how it moves around Titan," says Rannou. The spacecraft's Saturn tour was planned up to 2008, but may be extended to 2010. The next Cassini flyby is planned for 7 September 2006.